Pressure measuring glow plug

ABSTRACT

This disclosure relates to a glow plug comprising a housing tube, a glow element, which protrudes from the housing tube and is movable against a return force in the longitudinal direction of the housing tube, an inner pole connected to the glow element for applying supply voltage to the glow element, and a sensor for measuring a pressure acting on the glow element. According to this disclosure, a ground contact of the sensor, which is in the form of a tube or a sleeve, surrounds a section of the inner pole, and a signal contact of the sensor extends between the ground contact and a wall of the housing tube.

RELATED APPLICATIONS

This application claims priority to DE 10 2012 101 215.6, filed Feb. 15, 2012, which is hereby incorporated by reference in its entirety.

BACKGROUND

This disclosure relates to a glow plug and, more particularly, a pressure measuring glow plug.

Glow plugs comprising installed sensors are used to measure combustion chamber pressure. High temperatures, abrupt temperature changes and a strong mechanical load caused by vibrations and the like characterize the usage conditions of pressure sensors in glow plugs and make it difficult to precisely measure combustion chamber pressure.

The problem addressed by this disclosure is that of demonstrating a way to measure combustion chamber pressure with greater precision.

SUMMARY

In a glow plug according to this disclosure, a ground contact of the sensor is formed as a tube or sleeve and encloses a section of the inner pole circumferentially. The signal contact of the sensor extends between a section of the ground contact and the housing tube. Measuring accuracy is markedly improved as a result.

Within the scope of this disclosure it was found that, in the case of conventional pressure measuring glow plugs, pulse-width modulated voltage signals that are fed to the glow element via the inner pole can induce interference signals, which can impair the measuring accuracy of a pressure measurement. That is, the rapid on and off switching of the power supply unit can cause charges to be induced into a parasitic capacitor, which is formed by the inner pole and the sensor or the signal contact thereof. In a glow plug according to this disclosure, this can be prevented by a shield formed by the ground contact, in that the ground contact encloses the inner pole. In a glow plug according to this disclosure, the inner pole therefore forms a parasitic capacitor with the ground contact. Charges of this capacitor have no influence or only negligible influence on the sensor signal since they can flow to ground. Since the signal contact of the sensor is shielded from the inner pole by the ground contact, the measuring accuracy is improved.

The ground contact of the sensor can electrically contact the tube housing. Preferably, however, the ground contact of the sensor is electrically insulated from the tube housing. In this manner, electrical interference signals can be prevented to an even better extent. The inner pole can be connected to a potential connection of the glow element, and the tube housing can be connected to a ground connection of the glow element. The glow element and the sensor can therefore be at ground potentials that can differ from one another at least temporarily. An end section of the ground contact that contacts the sensor may be covered on the side thereof facing away from the sensor by an insulator, for example an insulator ring or an insulator disk. In this manner the ground contact can be electrically insulated from the tube housing in a technically simple manner.

The signal contact of the sensor can surround the ground contact of the sensor, for example in that the signal contact is in the form of a tube or sleeve. However, the signal contact can also be formed by one or more wires or strips disposed between the wall of the housing tube and the ground contact.

According to an advantageous refinement of this disclosure, the ground contact of the sensor protrudes at both ends thereof from the signal contact of the sensor. In this manner it is possible to shield the signal contact along the entire length thereof from the inner pole. The coupling-in of interference signals can thereby be reliably prevented.

According to a further advantageous refinement of this disclosure, the ground contact and/or the signal contact comprises an overhanging end section, i.e., a radially protruding end section, against which the sensor bears. Both the ground contact and the signal contact can have an overhanging end section. An overhanging end section can be provided, for example, as a flange at a tubular signal contact or ground contact. An overhanging end section can be used, advantageously, to establish reliable electric contact between the sensor and the end section. In particular, the sensor can be clamped between the overhanging end sections of the ground contact and the signal contact, thereby ensuring that reliable electric contact is maintained even in the presence of vibrations that occur during engine operation.

According to a further advantageous refinement of this disclosure, an insulation layer is located between the ground contact and the signal contact. The insulation layer can be in the form of a separate tube, for example. It is also possible for the ground contact to be wrapped with an insulator, for example a plastic film. An insulation layer may also be located between the inner pole and the ground contact. This insulation layer can also be in the form of a tube, an inner coating of the ground contact or a coating of the inner pole.

According to a further advantageous refinement of this disclosure, the inner pole, the ground contact of the sensor and the signal contact of the sensor protrude from the tube housing. In this manner the glow plug can be connected easily, namely by way of the inner pole to a voltage source and by way of the ground contact of the sensor and the signal contact of the sensor to a control device, which evaluates measured data on pressure delivered by the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of this disclosure are explained using an embodiment, with reference to the attached drawings, wherein:

FIG. 1 shows an embodiment of a glow plug in a partially exposed view;

FIG. 2 shows a detailed view of FIG. 1; and

FIG. 3 shows a schematic block diagram of the glow plug during operation.

DETAILED DESCRIPTION

The embodiments described below are not intended to be exhaustive or to limit this disclosure to the precise forms disclosed in the following detailed description. Rather, the embodiments are chosen and described so that others skilled in the art may appreciate and understand the principles and practices of this disclosure.

FIG. 1 shows a glow plug comprising a housing tube 1, from which a glow element 2 protrudes. The glow element 2 is fastened to the housing tube 1 by way of a diaphragm 3. When the glow element 2 moves in the longitudinal direction thereof, the diaphragm 3 generates a reset force. The strength of a combustion chamber pressure acting on the glow element 2 determines the extent to which the glow element 2 is pressed into the housing tube 1.

The glow element 2 can press directly onto a sensor 4 in order to measure pressure. In the embodiment shown, however, a tube 5 is mounted on the glow element 2, by way of which the glow element 2 presses onto the sensor 4. The inner pole 6 of the glow plug extends in the tube 5. The inner pole 6 is connected to the glow element 2 and is used to apply supply voltage to the glow element 2.

In the embodiment shown, the sensor 4 is a piezoelectric sensor. Other sensors could also be used in order to detect the position of the glow element 2 and, therefore, to measure the combustion chamber pressure. As shown in FIG. 2 in particular, the sensor 4 is in the form of a ring, through which the inner pole 6 of the glow plug extends. The sensor 4 is contacted by a ground contact 7 on a side facing the glow element 2. A side of the sensor 4 facing away from the glow element 2 is contacted by a signal contact 8. Both the signal contact 8 and the ground contact 7 are in the form of a tube or sleeve and are disposed around the inner pole 6. The sensor 4 is located between an overhanging end section of the ground contact 7 and an overhanging end section of the signal contact 8.

In the embodiment shown, the inner pole 6 protrudes by way of the two ends thereof from the ground contact 7 and from the signal contact 8. A tubular section of the ground contact 7 therefore surrounds a section of the inner pole 6 that extends in the tube 5 to the glow element 2. The tubular section of the ground contact 7 is enclosed by a tubular section of the signal contact 8. The signal contact 8 therefore surrounds, in the tube housing 2, a section of the ground contact 7 and a section of the inner pole 6.

The ground contact 8 is longer than the signal contact 7. The ground contact 8 therefore protrudes from the signal contact 7 at both ends. The ground contact 8, the signal contact 7 and the inner pole 6 protrude from the housing tube 1. An insulation layer is located between the inner pole 6 and the ground contact 8. An insulation layer is also located between the ground contact 8 and the signal contact 7, for example an insulator tube 9, which is preferably made of plastic.

The inner pole 6 is connected to a potential connection of the glow element 2, and the tube housing 1 is connected to a ground connection of the glow element 2. In an outwardly conducting glow element 2, this ground connection can be established by way of the diaphragm 3. The ground contact of the sensor 4 can be electrically insulated from the tube housing 1. In the embodiment shown, the overhanging end section of the ground contact 7 is located between the sensor 4 and an insulator disk 10. The insulator disk 10 is located between the sensor 4 and the tube 5, for example on an end surface of the tube 5, and thereby electrically insulates the ground contact 7. In a similar manner, the overhanging end section of the signal contact 8 is located between the sensor 4 and an insulator ring 11. The housing tube 1 is closed by a cap 12, which is used as an abutment for the pressure exerted by the glow element 1. The insulator ring 11 may bear against the cap 12, which is the case in embodiment shown.

FIG. 3 schematically shows a block diagram of the above-described glow plug together with a connected voltage source 13 and an electronics unit. In the example shown in FIG. 3, the electronics unit comprises an active low-pass filter, which is formed by an operational amplifier, the capacitor C_(f) and the resistor R_(f). The electronics unit can be disposed in the tube housing 1 of the glow plug, or outside thereof, can be used as a system component of signal conditioning or signal amplification that is separate from the glow plug. The electronics unit can also be integrated in the glow plug control device, for example. The design of the electronics unit can deviate from the example shown in FIG. 3. Preferably, however, the electronics unit contains amplifiers and/or filters.

The sensor 4 of the glow plug is contacted by the ground contact 7 on one side and by the signal contact 8 on the other side. The ground contact 7 and the signal contact 8 lead to the inputs of the operational amplifier shown in FIG. 3. The ground contact 7 and the signal contact 8 may also be routed to any other filter or evaluation circuit. The resistor R_(glow) is the glow element 2, which is connected to the voltage source 13 via the inner pole 6. The voltage source 13 supplies the glow element 2 with pulse-width modulated voltage signals. Together with the ground contact 7 of the sensor, the inner pole 6 forms parasitic capacitance indicated as C_(para) in FIG. 3. In this manner, the ground contact 7 is connected to the ground of a control device, which is indicated in FIG. 3 as GND_(ECU). The resistor R_(glow) of the glow element 2, however, is connected to the ground of the engine GND_(Engine), which does not necessarily have to be the same as the ground of the control device, but rather can deviate therefrom temporarily or continuously.

While exemplary embodiments have been disclosed hereinabove, this disclosure is not limited to the disclosed embodiments. Instead, this application is intended to cover any variations, uses, or adaptations of this disclosure using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.

REFERENCE NUMERALS

-   1 housing tube -   2 glow element -   3 diaphragm -   4 sensor -   5 tube -   6 inner pole -   7 ground contact -   8 signal contact -   9 insulator tube -   10 insulator disk -   11 insulator ring -   12 cap -   13 voltage source -   C_(f) capacitor -   R_(f) resistance -   R_(glow) resistance of the glow element -   C_(para) parasitic capacitance of inner pole and ground contact -   GND_(ECU) ground potential of control device -   GND_(Engine) ground potential of engine 

1-11. (canceled)
 12. A glow plug comprising: a housing tube; a glow element, which protrudes from the housing tube and is movable against a reset force in the longitudinal direction of the housing tube; an inner pole electrically connected to the glow element for applying a supply voltage to the glow element; a sensor for measuring a pressure acting on the glow element; a ground contact of the sensor, said ground contact comprising a tube or sleeve surrounding a section of the inner pole; and a signal contact of the sensor extending between the ground contact and a wall of the housing tube.
 13. The glow plug according to claim 12, wherein the sensor comprises a piezoelectric sensor.
 14. The glow plug according to claim 12, wherein the sensor comprises a ring surrounding the inner pole.
 15. The glow plug according to claim 12, wherein the ground contact is connected on a side of the sensor facing the glow element, and the signal contact of the sensor is connected on an opposing side of the sensor facing away from the glow element.
 16. The glow plug according to claim 12, wherein both the ground contact and the signal contact have an overhanging end section that contacts the sensor.
 17. The glow plug according to claim 16, wherein the overhanging end section of the signal contact is located between an insulator ring and the sensor.
 18. The glow plug according to claim 12, wherein the ground contact of the sensor is electrically insulated with respect to the tube housing.
 19. The glow plug according to claim 12, wherein the inner pole is connected to a potential connection of the glow element and the tube housing is connected to a ground connection of the glow element.
 20. The glow plug according to claim 12, wherein the signal contact of the sensor encloses the ground contact.
 21. The glow plug according to claim 20, wherein the ground contact of the sensor protrudes at both ends thereof from the signal contact of the sensor.
 22. The glow plug according to claim 12, wherein the inner pole protrudes at both ends thereof from the ground contact of the sensor. 